An anesthetic metering device of the above-described type in the form of an anesthetic vaporizer has become known from West German Patent No. DE-PS 12,71,903.
In the prior-art anesthetic vaporizer, the gas stream is split at a gas inlet opening into a bypass stream, which directly reaches the gas discharge opening via a bypass valve, and a vaporizer chamber stream, which is led through a vaporizer chamber, is enriched with anesthetic in the vaporizer chamber, and is again fed into the bypass stream behind the bypass valve via an adjusting device. The bypass valve is designed as a pipe throttle and generates in the bypass stream a flow resistance that is a measure for the split ratio of the gas streams through by bypass valve and the vaporizer chamber.
The adjusting device is designed as a cone valve performing stroke movements, with which the anesthetic concentration at the gas discharge opening is adjusted. Under laminar flow conditions, the anesthetic concentration value preselected at the adjusting device depends only slightly on the value of the total stream, since the flow resistance at the bypass valve changes in proportion to the gas stream, and the ratio of the bypass stream to the vaporizer stream consequently changes in the same manner. To compensate for temperature effects, a temperature stabilizer, which is designed as a water reservoir and stabilizes the anesthetic to the temperature T, is provided in the vaporizer chamber. Within the vaporizer chamber, the vaporizer chamber stream is enriched with anesthetic up to the saturation concentration value determined by the temperature T. Thus, on uniting the vaporizer chamber stream with the bypass stream, a maximum anesthetic vapor volume that can be metered is set by the saturation concentration value.
One disadvantage of the prior-art anesthetic vaporizer is the fact that it can be used only to meter an anesthetic whose boiling point is markedly above the operating temperature. If this condition is not met, anesthetic vapor will enter the bypass stream due to boiling anesthetic, as a result of which undefined concentration ratios will appear, which make accurate metering impossible.
In a metering device for anesthetic gases known from Austrian Patent No. AU-PS 58,523/73, the gas stream--here, oxygen--is passed through a vacuum-generating venturi tube acting as a bypass valve, and the anesthetic gas is fed into the stream by means of a diaphragm-controlled valve acting as an adjusting device, as a function of the vacuum becoming established over the venturi tube. The vacuum relative to the atmospheric pressure serves as the regulated quantity for metering on the diaphragm-controlled valve.
It is disadvantageous in the metering device that no provisions are made for accurately setting the concentration, and the adjustment of the amount of anesthetic gas metered is possible only as a function of a vacuum relative to the atmospheric pressure, rather than in proportion to the oxygen gas stream. In addition, pressure variations at the gas discharge opening, as a consequence of, e.g., the respiration pressure, directly affect the vacuum at the venturi tube and consequently the metering of the anesthetic gas as well.